1. Field of the Invention
The present invention relates to an improved washing and/or rinsing process in an automatic dishwashing machine of the type comprising a wash tub in which at least lower and upper baskets are provided for supporting crockery to be washed.
2. Description of the Related Art
Conventional dishwashers are known to have a plurality of baskets each associated with a respective rotary spray arm which is fed with water by a pump.
As described in EP-A-0 237 994, for instance, the spray arms are preferably supplied with water alternately, rather than concurrently. This allows the motor and the fluid circuit through which water is fed by the pump to be compact, and also reduces the noise produced by the water jets impinging on the crockery. To this aim, the spray arms are connected to the delivery section of the pump through respective supply conduits which are associated with a monostable flow distributing valve, the latter being indirectly controlled by the programmer of the machine to perform a washing cycle in which the spray arms are cyclically alternately operated. In particular, the flow distributing valve is controlled by repeatedly discontinuing, with different idle times, the energization of the motor driving the circulating pump.
Although this solution is particularly advantageous and precise in operation, the number of commutations occurring during every washing cycle may in the long run shorten the life of the motor.
In addition, when the resistive heating element of the water heater, that is positioned on the bottom of the wash tub, is energized during the operative phases in which only the upper spray arm is supplied with water, the element is easily overheated and therefore damaged, giving off bad odors. In fact, because the heating element is located relatively far from the upper spray arm, it is only insufficiently sprinkled by the water which is in suspension in the wash tub during such operative phases. This drawback, which for dimensional reasons does not arise when the lower spray arm is operated, may be overcome by increasing the amount of water fed into the tub. However, this compromises the current demand for energy saving devices.
As an alternative, the heater may be associated with special means capable of collecting the water circulated in the tub and keeping the resistive element wet with such water. This solution is only partially effective and in any case undesirably complicates the structure of the bottom part of the wash tub, therefore affecting the positioning of the various parts and increasing the manufacturing costs of the machine on an industrial scale.
Dishwashers are also known in which the resistive heating element is located at the delivery or suction side of the pump, so as to be permanently immersed in water when it is energized. This solution, however, involves remarkable complications in the structure and assembling of the machine. Furthermore, to prevent parts made of plastic which are adjacent to the resistive element from being damaged, it is necessary to provide temperature and/or level sensing devices capable of controlling the actual operating temperature of the heater and/or water level. The provision of such devices further complicates the structure of the whole dishwasher. Moreover, with such a solution it is obviously impossible to use the same heater to perform, as is traditional, a final drying phase of the crockery under "dry" conditions.